In typical conventional propulsion systems a prime mover, for example a main engine, inside a vessel's hull provides power, either directly or by driving a generator supplying electrical power to an electric motor, to a propeller shaft extending from the inside of the hull, through the hull, to a propeller mounted on the shaft outside the hull. This type of propulsion is reliable. However the prime mover and propeller shaft require a substantial amount of space within the vessel's hull and thus diminish the cargo capacity of the vessel. Further, the propeller shaft, which needs to be fairly long in order to extend from the inside of the hull to the outside of the hull, must be supported by several bearings, which increases the cost and maintenance requirements of this type of propulsion system. Due to the space required, this propulsion system is very difficult to retrofit to a hull.
An alternative propulsion system, which is commonly used in the offshore industry for supply vessels and service vessels, is the azimuth thrusters propulsion system. In this system an azimuthing pod bearing a propeller is rotatably mounted to the hull, thus allowing the pod, and thereby the direction of the thrust provided by the propeller, to be controlled for both propelling and steering the vessel. Typically, the propeller on the pod is driven via an angle gear by an electric motor in the stern of the hull above the pod. Alternatively, the electric motor may be provided directly in the pod. Electric power to the electric motor is supplied by a generator driven by a prime mover, for example a diesel engine.
This type of propulsion makes the vessel very manoeuvrable and obviates the need for a rudder and a lateral thruster in the stern. Furthermore, it does not diminish cargo space, since all components are mounted in the stern. However, the system is vulnerable due to mechanical complexity. Also vessels operating in ice observe problems, unless the thrusters are very much designed for this special purpose only. In case of breakdown or major maintenance, docking of the vessel is often required because many of the components of the propulsion system, for example the angle gear and sealings, are in fact not accessible for repair or service while at sea. This is a problem in the offshore industry where supply vessels and service vessels are very costly to take out of service outside the normal docking. Moreover, the increasing exploration in arctic areas calls for more heavy-duty systems that can withstand ice floes as well as perform ice breaking.